1. Field of the Invention
The present invention relates to a gamma-reference-voltage generating circuit and an apparatus for generating gamma-voltages and a display device having the circuit. More particularly, the present invention relates to a gamma-reference-voltage generating circuit having a thermal compensation feature, an apparatus for generating gamma-voltages having the gamma-reference-voltage generating circuit and a display device including the gamma-reference-voltage generating circuit.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) device displays an image by providing a pixel with an analog gradation voltage using a thin-film transistor (TFT). The analog gradation voltage is a data voltage that is provided to a data line of the LCD device.
When a gate voltage of a high level is applied to the thin-film transistor, the thin-film transistor is turned-on, so that the data voltage is charged in a pixel that is defined by a liquid crystal capacitor and a storage capacitor. The variation of the voltage difference between the charged data voltage and a common voltage changes the transmittance of the light passing through a liquid crystal layer, and thus desired gradations are displayed.
When the high value of the gate voltage that is applied to a thin-film transistor of the pixel, the pixel voltage reaches the data voltage. However, the pixel voltage drops by as much as a kickback voltage due to parasitic capacitors of the thin-film transistor after the gate voltage becomes low.
The kickback voltage varies significantly depending on the voltage difference between the pixel voltage and the common voltage as well as depending on the pixel voltage itself. It is because the capacitance of the liquid crystal capacitor depends on the voltage across the liquid crystal capacitor due to the dielectric anisotropy of the liquid crystal. Therefore, the liquid crystal capacitance is varied in accordance with a white gradation displaying and a black gradation displaying, so that the kickback voltages corresponded to each gradations is varied.
Due to the difference of kickback voltages of each gradation, when an image corresponding to a new pattern is displayed after a predetermined image is displayed for a long time, this causes a defect in the LCD device referred to as image sticking.
In order to remove the image sticking, for example, a method of decreasing a remaining DC voltage may be used, which is calculating a kickback voltage corresponding to gradations and compensating a gamma using the calculated kickback voltage.
In order to remove the image sticking, for another example, a method of reducing a difference between kickback voltages corresponding to gradations may be used, which is decreasing an absolute value of the kickback voltages.